In sophisticated hydraulic circuits of e.g., cranes, concrete distributing booms and other load lifting and manipulating units, proportional directional spool valves are normally used for allowing a plurality of consumers to be operated simultaneously. In practical use, it is frequently necessary to control a plurality of consumers completely individually and at the same time, and this control should be effected such that it is independent of the load pressure to the highest possible degree.
From the HAWE product overview 2011, pages 98 to 101, a proportional directional spool valve, type PSL, with connection blocks and an ancillary block is known. With respect to FIG. 1a, a hydraulic circuit is schematically shown, in which two known proportional directional spool valves, type PSL, designated by reference symbols PS1, PS2 in the figure, with a suitable connection block for a constant delivery pump 1, are provided for the operation of two consumers V1, V2. A supply pressure P outputted by a constant delivery pump 1 is fed through a supply line 6 to a plurality of consumers V1, V2, e.g., hydraulic cylinders, via the respective proportional directional spool valves PS1, PS2 for driving the consumers V1, V2, which are shown in a highly schematized representation. The inflow upstream of the proportional directional spool valves PS1, PS2 is controlled by a respective pressure-controlled 2-way directional control valve 41, 42 disposed upstream of each proportional directional spool valve PS1, PS2 in the inflow direction. If, during operation, at least one of the consumers V1, V2 is to be operated, the proportional directional spool valve PS1, PS2 associated therewith is deflected upwards or downwards from the shut-off condition shown, depending on whether a connection A1, A2 or B1, B2 connected to the respective consumer V1, V2 is to be connected to the supply line 6. By means of the deflection stroke, a volumetric flow to the respective consumer V1, V2 is predetermined by the proportional directional spool valves PS1, PS2.
Via a suitable LS duct LS1, LS2 (shown by a broken line in FIG. 1), a load pressure dropping downstream of the respective proportional directional spool valve PS1, PS2 is signaled to the associated 2-way directional control valve 41, 42. In addition, it is guaranteed by means of shuttle valves 2 that the highest load pressure among the load pressures signaled by the LS ducts LS1, LS2 (corresponds to the load pressures dropping across the consumers V1, V2) is signaled to a circulation regulator 8 of the constant delivery pump system.
The load pressure signaled by the LS ducts LS1 and LS2 to the 2-way directional control valves 41 and 42 is—supporting the pre-load of the pre-load spring—applied to the 2-way directional control valve 41 and 42, respectively, such that it acts in the opening direction of the 2-way directional control valve 41 and 42. In addition, a pressure signal tapped off at the output side of the respective 2-way directional control valve 41, 42 is applied to each 2-way directional control valve 41, 42 in the closing direction (i.e., counteracting the pre-load) of the respective 2-way directional control valve 41, 42. Each of the 2-way directional control valves 41, 42 is pre-loaded in the opening direction by a pre-load spring so that the 2-way directional control valves 41, 42 are open in the idle state.
It follows that, in the condition of equilibrium, a specific pressure difference will occur between the tapped LS pressure and a pressure signal corresponding to the tapped output-side pressure of the 2-way directional control valve. Changes in the volumetric flow are thus controlled to a constant value in the case of springs having a small spring constant or a flat spring characteristic. Hence, each of the 2-way directional control valves 41, 42 controls a volumetric flow through the respective proportional directional spool valve PS1, PS2 to a constant value in a load-independent manner. In other words, if the volumetric flow occurring downstream of the proportional directional spool valve PS1 or PS2 decreases during operation of the consumer V1 or V2, also the pressure difference between the pressure dropping downstream of the 2-way directional control valve 41 or 42 (signaled as “pA”) and the load pressure dropping downstream of the proportional directional spool valve (signaled as “pLS” to the 2-way flow control valve 41 or 42 via the LS duct LS1 or LS2) will decrease, so that a control piston (not shown) will shift in the 2-way directional control valve 41 or 42 along the opening direction. The consequence is that an equilibrium of forces is reestablished at the control piston in the 2-way directional control valve 41 or 42, although at a larger throttle cross-section in the 2-way directional control valve 41 or 42, so that the reduction of the volumetric flow and of the pressure difference (pA−pLS) is compensated for. The volumetric flow to the consumer and the pressure difference between the pressure provided by the constant delivery pump 1 and the pressure in the load circuit to the consumer are thus controlled to a constant value. If, however, the pressure difference (pA−pLS) at the proportional directional spool valve PS1 or PS2 increases, which corresponds to an increase in the volumetric flow, the 2-way flow control valve will be controlled in the closing direction until a new equilibrium of forces is established. In the 2-way directional control valve a control piston (not shown) shifts in the closing direction whereby a throttle cross-section in the 2-way directional control valve 41 or 42 is reduced. A reduction of the throttle cross-section, however, means that the volumetric flow and the pressure difference (pA−pLS) will decrease (counteracting the initial increase) until an equilibrium of forces is reestablished.
In FIG. 1b the resultant characteristic of the 2-way directional control valve 41, 42 is shown in a schematic representation, in which a volumetric flow Q (along the x-coordinate in arbitrary units) is plotted against a pressure difference Δp (corresponds to a pressure difference of pump pressure—pLS) (along the y-coordinate in arbitrary units). After an initial triggering curve section AK, the characteristic shown in FIG. 1b exhibits a control curve section RK with a vertical profile, which stands for the independence of the volumetric flow from the pressure difference Δp when an equilibrium of forces is established, since, in the control curve section, the volumetric flow is controlled to a constant value Q0 independently of the pressure difference Δp.
It is an object of the present disclosure to provide a volumetric flow control, which deviates from the above described load-independent volumetric flow control and which, for improving the variability of hydraulic circuits, allows a load-dependent volumetric flow control depending on the specific case of use. For example, in order to accomplish a “good operational feeling” a load-dependent control of the volumetric flow may, in some cases of use, definitely be desirable so as to impart to the user a “feeling for the load”.
It is e.g., an object of the present disclosure to provide a 2-way flow control valve and a valve assembly comprising such a 2-way flow control valve, which stabilize the volumetric flow control at the operating points, especially in the case of interaction with other hydraulic controllers, and/or allow a more precise volumetric flow control in the low differential pressure range.